This invention relates generally to an improved excitation technique for an electromagnetic flowmeter, and more particularly to a method and a system based thereon to effect automatic control of the frequency of the excitation current supplied to the electromagnet of the flowmeter so that it is higher in a high flow-rate range and lower in a low flow-rate range.
In an electromagnetic flowmeter, a magnetic field is established by an electromagnet having an excitation coil, the field being intercepted by a fluid passing through a flow tube to induce a signal between a pair of diametrically-opposed electrodes, which electrode signal is indicative of flow rate.
In flow rate measurement utilizing a magnetic flowmeter, it has heretofore been the practice to make use of an a-c excitation current in order to eliminate polarization effects which take place between the electrodes and the fluid being metered. However, when employing the a-c magnetic field, eddy currents are generated that are 90.degree. out of phase with the magnetic flux.
Although, in an ideal state, these eddy currents in the cross-sectional plane of the flow tube which includes both electrodes, flow symmetrically with respect to the tube's axis, in actual practice these currents are asymmetrical with respect to this axis because of an unbalance in the geometry of the tube. This gives rise to unbalanced eddy currents which are changed by the capacitance between the electrodes and the fluid into components that are in-phase with the detected signal. These in-phase components result in zero drift, in that the eddy currents fluctuate from time to time.
With a view to overcoming this drawback, it has heretofore been the practice to provide an excitation system in which the excitation frequency is decreased to a value below the frequency of the commercial power line in order to reduce the level of eddy currents generated in the flowmeter. However, when the excitation frequency is so decreased, it becomes impossible to measure the flow rate of fluids whose velocity lies in a high flow-rate range.